## Description
Fix #8769 for mismatched track kind should be plural descriptions
according to definition of `AudioTrackKind`
#8769

## Specific Changes proposed
Corrects `description` to `descriptions`

## Description
Adds a `TransientButton` component for the types of button that are
shown on top of the video briefly during playback and reappear when
there is user activity. e.g. Unmute buttons, skip intro. It aims is to
be a generic button type to be extended. Some basic styles are provided
but kept light to not complicate customisation.
It's important to insert a transient button before the control bar for
the tab order to make sense.

_Optionally_ takes focus when shown.

## Specific Changes proposed
Adds `TransientButton` component.

## Requirements Checklist
- [x] Feature implemented / Bug fixed
- [ ] If necessary, more likely in a feature request than a bug fix
- [x] Change has been verified in an actual browser (Chrome, Firefox,
IE)
  - [x] Unit Tests updated or fixed
  - [ ] Docs/guides updated
- [x] Example:
https://deploy-preview-8629--videojs-preview.netlify.app/sandbox/transient-button.html
- [x] Has no DOM changes which impact accessiblilty or trigger warnings
(e.g. Chrome issues tab)
  - [x] Has no changes to JSDoc which cause `npm run docs:api` to error
- [ ] Reviewed by Two Core Contributors

## Description
See issue #8653 

## Specific Changes proposed
When in `middleware.js` the function `clearCacheForPlayer` runs, before
setting a value to null in middlware caches, it checks if the key exists
in the first place.

## Requirements Checklist
- [x] Feature implemented / Bug fixed
- [x] If necessary, more likely in a feature request than a bug fix
- [x] Change has been verified in an actual browser (Chrome, Firefox,
IE)
  - [ ] Unit Tests updated or fixed
  - [ ] Docs/guides updated
- [x] Example created ([starter template on
JSBin](https://codepen.io/gkatsev/pen/GwZegv?editors=1000#0))
- [ ] Reviewed by Two Core Contributors

---------

Co-authored-by: Giuseppe Piscopo <g.piscopo@braincrumbz.com>
Co-authored-by: mister-ben <git@misterben.me>

- Removes erroneous `@return {Component}` from `ReadyCallback` in
Component
- Remove `@this {Component}` from `ReadyCallback` in Component, because
this is not accurate for classes that extend Component
- Adds `@returns {void}` to `ReadyCallback` in Component
  - Produces a more accurate typedef
- Isn't strictly accurate in jsdoc/javascript but does this matter since
it's well understood. Absence of `@return` is interpreted by tsc as
returning `{any}`, `@returns {undefined}` would require an explicit
`return undefined` in ts.
- Adds a `{PlayerReadyCallback}` in Player with `@this {Player}` used in
the `new Player()` and `videos()` constructors.
  - Are we ok adding this new typedef
- Is inconsistent with `player.ready()` which uses `ReadyCallback`
without `@this` - but this can't be changed without adding an otherwise
unnecessary override just to pander to tsc.

## Requirements Checklist
- [x] Feature implemented / Bug fixed
- [ ] If necessary, more likely in a feature request than a bug fix
- [x] Change has been verified in an actual browser (Chrome, Firefox,
IE)
  - [ ] Unit Tests updated or fixed
  - [ ] Docs/guides updated
- [ ] Example created ([starter template on
JSBin](https://codepen.io/gkatsev/pen/GwZegv?editors=1000#0))
- [x] Has no DOM changes which impact accessiblilty or trigger warnings
(e.g. Chrome issues tab)
  - [x] Has no changes to JSDoc which cause `npm run docs:api` to error
- [ ] Reviewed by Two Core Contributors